This invention relates to an injection molding process, and in particular, a method and apparatus for reducing the size of a core layer hole in an injection molding process.
Presently, many plastic articles are formed by injection molding processes. These articles include common items such as test tubes and pre-forms for forming items such as beer and ketchup bottles. Many of these articles are produced from injection molding machines having the ability to inject multiple plastic layers at the same time (i.e., co-injection). Thus, the injection-molded articles may have two or more layers of plastic in their final form (i.e., multi-layer plastic articles).
A common configuration of multi-layer plastic articles includes an interior or xe2x80x9ccorexe2x80x9d plastic layer which is surrounded on all sides by an outer plastic layer. For example, see U.S. Pat. Nos. 5,914,138 and 6,187,241, both assigned to Kortec, Inc. The disclosures of both of these patents are incorporated herein by reference. Typically, the interior (core) layer is formed of a material such as Ethyl Vinyl Alcohol (EVOH), and the outer layer is formed from a material such as Polyethylene Terephtholate (PET). This construction produces a sandwich structure wherein the outer layer (e.g., PET) forms both the exterior and the interior of the article, and the interior (core) layer (e.g., EVOH) is sandwiched therebetween.
However, a common problem experienced when injection molding such articles is that a hole or gap is formed in the interior (core) layer at the base of the molded article where the interior (core) layer enters the mold. The hole is formed because the interior (core) is formed by an annular stream with a diameter that decreases towards the base of the molded article. The diameter of the annular core stream at the base of the article corresponds directly to the diameter of the hole or gap. In particular, the interior (core) layer enters the mold as an annular stream which is surrounded on both sides by inner and outer covering layers. When the flow of the interior (core) layer is stopped, a tail of the interior (core) layer continues up the sidewall of the molded part, thereby creating a hole at the base of the molded part which is typically much larger than a gate of the nozzle which injects the interior (core) and outer layers.
FIG. 14 shows a conventional injection molding system 500 which includes injection molding apparatus 510 and a mold 550. The injection molding apparatus 510 includes a nozzle 515 which has various passageways for transferring plastic materials to the mold 550. A first series of passageways 520 are used for delivering plastic material 521 to the mold 550. Plastic material 521 forms both an inner covering layer (IL) and an outer covering layer (OL). A second series of passageways 525 are used for delivering an interior annular layer (IA) of plastic material 526 to the mold 550. The interior annular layer IA may be, for example, a barrier layer that prevents passage of gases into or out of the molded article. The first and second series of passageways 520, 525 come together at a gate portion 530 of the nozzle 515. The nozzle gate portion 530 comprises a relatively narrow portion of the nozzle 515 which feeds directly into the mold 550. The injection molding apparatus 510 also includes a throttle pin 535 for controlling the flow of plastic material (521, 526) through the nozzle gate portion 530.
Particularly, if the interior (core) layer IA is stopped too soon, the interior (core) layer IA will travel up the sidewall of the molded part, thereby creating a large hole or gap 595 at the base of the part. If the interior (core) layer IA is stopped too late, some interior (core) material IA will be left in the nozzle 530 of the injection molding apparatus 510. This remaining material will contaminate the next molding by flowing into the next molded part and possibly ending up on an outside wall of the part. The result of this type of contamination is often referred to as xe2x80x98scale.xe2x80x99 Scale can occur inside and/or outside of the molded part.
Controlling the size of the hole or gap created by the annular interior (core) streams is fundamental in present day injection molding systems. If this gap is too large, the barrier properties of the molded part will be significantly reduced. In other words, a vacuum created within a substance-containing portion of the part cannot be maintained for a long period of time because exterior gases will enter the part through the hole, or conversely pressure can not be maintained in the part because gases within the molded part will seep out through the hole.
Thus, there is presently a need for a method and apparatus for injection molding articles where the size of a gap or hole in the interior plastic layer is efficiently controlled.
The present invention is a method and apparatus for extruding plastic articles, the method comprising the steps of injecting at least one stream of plastic material into a mold, the mold including a first portion and a second portion, the first portion of said mold being used for forming at least one article, and the second portion of said mold forming a sprue attached to the at least one article, and, terminating the at least one stream of plastic material in the second portion.
The above and other advantages and features of the present invention will be better understood from the following detailed description of the exemplary embodiments of the invention which is provided in connection with the accompanying drawings.